Cleaning of metals



operation by the known Patented June 8, 1948 CLEANING OF METALS Norman L. Evans, Northwich, England, assignor to Imperial Chemical Industries Limited, a corporation of Great Britain No Drawing. Application August 16, 1946, Serial 1 Claim.

This invention relates to the cleaning of metals .and more particularly to a method for cleaning metals in fused baths.

In U. S. patent application Serial No. 469,703, filed Dec. 21, 1942, now Patent No. 2,380,284 issued July 10, 1945, is described a process by which metals can be descaled by treatment in a substantially anhydrous fused bath containing alkali metal hydroxide and alkali metal cyanide. Such a process is preferably operated using 20-80% by weight of sodium cyanide dissolved in fused sodium hydroxide, and the bath preferably contains less than 0.2% of water by weight. The treatment is usually carried out at 300-500 C.

I have now found that improved descaling is obtained by a modification of the above procedure, involving smaller amounts of cyanide and the presence of small but definite amounts of water, whereby efiervescence occurs in the bath and causes a scouring which lifts the reduced oxide and leaves a clean metal surface.

According to the present invention, I provide a process for cleaning metals and metal articles which comprises immersing metals other than zinc and zinc alloys, or articles thereof, in a bath containing 1-10% by weight of alkali metal cyanide, 01-20% by weight of water, and the remainder principally alkali metal hydroxide, at a temperature of 300-600 C. for between /2 and 30 minutes.

Commercially available alkali metal cyanides and hydroxides in the form as manufactured Or after short periods of storage normally contain small amounts of water, of the order of magnitude of 0.2% by weight. Such amounts are within the scope of this invention. The bath will normally also contain alkali metal carbonate formed during use, and minor proportions of this and other substances may be added if desired. For comparison with the efiervescence and cleaning obtained by the process of the present invention, process with anhydrous baths which are obtainable by dehydrating com- Great Britain September 7,

merci-ally available materials with sodium metal, sodium monoxide, sodium hydride or other such materials, does not give efiervescence and does not lift the reduced oxide film from the cleaned metal. Quenching and acid pickling is relied on in the prior processes as a means of lifting the reduced oxide film, but the present invention provides improved removal and simplified operation wherein acid pickling is unnecessary except in the case of high chromium and other similar alloys.

The 1-l0% by weight of alkali metal cyanide used in the baths described herein is sufiicient to act as a reducing agent under the present conditions, and thereby reduce the oxide scale present on the metal. 'When using the baths according to this invention it is desirable to arrange for the fumes to be removed from the atmosphere near to the bath, for example by a ventilating hood. The preferred baths contain 2-5% by weight of sodium cyanide, 0.31% of water, and the remainder substantially sodium hydroxide with small amounts of sodium carbonate which may be formed during the operation and generally not exceeding 10%.

What I claim is:

A process for-cleaning metals and metal articles which comprises immersing metals other than zinc and zinc alloys, or articles thereof, in a fused bath containing 1-10% by weight of alkali metal cyanide, 0.1-2.0% by weight of water, and the remainder principally alkali metal hydroxide, at a temperature of 300600 C. for between /2 and 30 minutes.

NORMAN L. EVANS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

